Multi-mode mobile stations are commonly available. During the use of a multi-mode mobile station it may be the case that the two supported cellular systems operate in the same center frequency band (for example, Wideband CDMA (WCDMA) 1900 MHz and the Global System for Mobile Communications (GSM)1900 MHz.) Also, the IS-95 and TDMA systems may all operate in the same frequency band. Furthermore, in the 850 MHz band there are the AMPS, 850 TDMA, 850 GSM and CDMA systems
In a typical case one antenna is used for transmitting and receiving in both of the cellular systems. As an example, and referring to the dual mode system 1 shown in FIG. 1, the WCDMA 1900 MHz and the GSM 1900 MHz RF (Radio Frequency) subsystems, which may be embodied in the same ASIC or in separate ASICs 4 and 5, can each have their own transmit (TX) chain and associated power amplifier (PA). The working assumption has been that only one antenna 2 is used that has a single feed point (FP). For this to be true the GSM and WCDMA branches are coupled to the antenna 2 by an antenna switch (SW) 3.
The use of one antenna has been preferred due to the fact that the radiation pattern has been found to be impaired when there is another antenna nearby that operates on the same frequency. This case is shown in FIG. 2, where it is assumed that the WCDMA 1900 antenna 2A is transmitting, and that the GSM 850/1900 antenna 2B on the same mobile station is coupling in undesired energy from the WCDMA 1900 antenna transmission. The result is that the electromagnetic energy propagation pattern of the WCDMA 1900 antenna 2A is interfered with. The same situation can exist when the antenna 2B is transmitting, and undesired energy is coupled into the antenna 2A.
Referring again to FIG. 1, since the antenna switch 3 can be damaged by high voltage spikes, an electrostatic discharge (ESD) protection device 6 is typically installed between the switch 3 and the antenna 2. However, the use of the switch 3 and the ESD device 6 results in a loss of about 1 dB (0.5 dB loss in each of the switch 3 and the ESD device 6) in all modes of operation.
To reiterate, the use of two same frequency-tuned antennas in the prior art has proved to be difficult because of the coupling phenomenon between two same-resonance frequency antenna circuits. The coupling phenomenon has been known to reduce the antenna radiation gain. As a result, it is also known to use a single antenna 2 that is selectively coupled to either the first mode circuitry or the second mode circuitry through the switch 3. However, the use of the switch 3 also requires the presence of the ESD device 6 to prevent the switch from being damaged by static electric discharges coupled in through the antenna 2. Unfortunately, the use of the switch 3 and the ESD device 6 inserts a loss about 1 dB in all modes of operation, including the receive mode of operation. For example, the increased losses can result in a reduction in receiver sensitivity (by about 1 dB), as the switch 3 and ESD device 6 function to attenuate the already low level received signal.
It has further been found that by incurring additional losses after the power amplifier, such as by providing the switch 3 and ESD device 6, the efficiency of the power amplifier is reduced. This reduction in efficiency results in an increase in current consumption and a consequent reduction in battery life, user talk-time and mobile station stand-by time. Also, the additional losses result in the generation of a higher operating temperature. The higher operating temperatures can be manifested by a noticeable heating of the case of the mobile station, by power amplifier reliability problems related to increased junction temperatures, and also by a drift in the frequency responses of the RF components.